Modern optical communications systems include network nodes interconnected by optical waveguides, typically being single-mode optical fibre links. Within the network nodes, communications signals are converted between electrical formats, used for signal processing including regeneration and routing, and optical formats, used for transmission between nodes. The links between nodes include multiple concatenated optical components, typically including multiple optical fibre spans, each being some tens of kilometers in length, and corresponding optical amplifiers overcoming the attenuation experienced by optical signals during transmission through the fibre spans.
Signals transmitted through optical fibre links are affected by linear dispersion processes, such as chromatic dispersion and polarisation mode dispersion. Chromatic dispersion, in particular, may be compensated at any convenient point in a transmission system, and many existing installed optical fibre transmission links include dispersion compensating components, such as lengths of dispersion compensating fibre, installed at network nodes and/or amplifier locations. The technique of deploying inline dispersion compensation components within an optical transmission system is sometimes known as “dispersion management”, and the resulting cumulative dispersion characteristic within the transmission links as a “dispersion map”.
While dispersion management techniques can be very effective, they suffer from the disadvantage of a lack of flexibility. For example, a dispersion map that is effective for a particular wavelength channel within a specific transmission link may be less effective for channels transmitted at other wavelengths, and/or if all or part of the link is subsequently incorporated into a different optical transmission path. Accordingly, upgrading dispersion-managed links to carry larger numbers of wavelength division multiplexed (WDM) channels, or reconfiguring the optical network in which the link is installed, may require a redesign of the dispersion management strategy, and modification of dispersion compensation components within the link. Greater flexibility may be achieved through the use of electronic dispersion compensation methods, such as that described in U.S. patent application Ser. No. 12/089,571 (also published as International Publication No. WO 2007/041799), which discloses in particular an electronic dispersion compensation method which uses block coding of information, and frequency domain equalisation of the resulting received signal, in order to provide complete compensation of linear dispersion in the electrical domain. This approach may particularly conveniently be implemented using orthogonal frequency division multiplexing (OFDM) methods for the encoding and decoding of the electrical signals. Nonetheless, there are a significant number of existing installed transmission systems which employ fixed dispersion management.
In addition to linear processes, propagating optical signals may be subject to nonlinear effects. While the levels of optical nonlinearity existing in most practical transmission media, and in silica optical fibres in particular, are relatively low, it is generally desirable to transmit optical signals at high power levels in order to maintain sufficient signal-to-noise ratios over extended transmission distances. The use of high transmission powers increases the effect of optical nonlinearities, resulting in optical signal distortion which ultimately limits the received signal quality, and thus the maximum transmission distance achievable before the signal must be detected, recovered and regenerated. It is therefore desirable to mitigate the effects of nonlinear distortion, as well as the effects of linear processes such as dispersion, as far as possible. U.S. patent application Ser. No. 12/445,386 (also published as International Publication No. WO 2008/074085) discloses methods and apparatus for compensating the effects of optical nonlinearities, via electrical signal processing at the transmitting end (ie pre-compensation) and/or at the receiving end (ie post-compensation). This prior disclosure is generally directed to compensating so-called single channel effects, such as self phase modulation (SPM), which result from high transmission power levels in a particular channel causing nonlinear distortion to the channel itself. However, in WDM systems in which information is transmitted using a large number of different wavelength channels, the contribution of a single channel to the overall optical power is relatively small, and so-called cross-channel effects such as four wave mixing (FWM) and cross-phase modulation (XPM) may be significant.
It is believed that inline dispersion compensation, such as the dispersion management techniques employed in many existing optical transmission links, may increase the levels of nonlinear distortion. This increased distortion is caused by enhanced nonlinear mixing between WDM channel, eg due to XPM, because the “walk off” between WDM channels is reduced through the use of dispersion management. In particular, dispersion causes different WDM channels to propagate through the optical fibre at different speeds, whereby an “averaging” effect reduces the severity of nonlinear distortions, since no portion of the signal is continuously subjected to any particular peak in overall optical power. However, in the presence of inline dispersion compensation, or dispersion management, this benefit is somewhat reduced.
It is generally considered that cross-channel effects, such as XPM, cannot be effectively mitigated by electronic processing, since reversing the effects of nonlinear propagation requires equalisation based upon a complete representation of all of the channels transmitted within the fibre, and their propagation characteristics. On the other hand, as has been noted above, the use of optical compensation techniques generally limits the flexibility of the network, particularly in relation to performing upgrades and/or reconfiguration in a simple and cost-effective manner.
It is accordingly an object of the present invention to provide improvements in the compensation and/or mitigation of nonlinear distortions, and in particular cross-channel effects, in a manner which can be effectively and efficiently applied to existing transmission links, including those employing dispersion management strategies, while also being adaptable to support network upgrades and reconfiguration.